The present invention relates to the use of fuzzy logic based integrated intelligent control of a wind turbine electrical generation system to improve its efficiency and performance. Wind-generated electricity in regions with good resources could have a profound impact on energy production industries in the U.S. and around the world.
It has been conservatively estimated that the U.S. accessible wind resource could produce more than 10 times the electricity currently consumed. For example, the Bonneville Power Administration has identified 20 to 60 gigawatts of potential wind capacity in eastern Idaho and Montana, alone. Wind energy is the most cost-competitive renewable energy technology for the bulk power market. It is estimated that over 60 billion kWh per year of electricity will be produced by wind energy by the year 2000. Thus, every 1% improvement in wind turbine generator efficiency will create a significant amount of energy production. Improvements of as high as 15-20%, which are possible with the present invention, are thus extremely desirable. Since fossil fuels are the primary energy source for the production of electricity in the U.S. and worldwide, a reduction in power production from these sources can result in a concomitant reduction of emissions of gases and pollutants which are considered to be acid rain precursors and contributors to the greenhouse effect (global warming). The pollutants include SO.sub.2 and CO.sub.2. The potential for enhanced environmental quality is realized because energy produced by environmentally clean wind turbines would be used to off-set energy produced by fossil fuels such as coal. Increasing the energy capture of wind turbines would also result in the reduction in the cost of wind produced electricity. Cheaper energy would enhance economic productivity, international competitiveness of U.S. industry, and cost savings from reduced energy bills. Wind energy production would also reduce the U.S. dependency on energy imports, particularly oil, as non-renewable resource, and improve national security.
The general components of wind turbines have changed little over the years. A rotor, rotatably supported by a tower, generates aerodynamic forces to turn a main shaft. Gears in the power train drive the generator at a given speed. Mechanical controls are provided to adjust the rotor speed in high winds and to keep the rotor facing into the wind. In the quest for higher rotor speeds, reduced blade area, and lower cost, new wind turbines are designed with flexible, lightweight blades, teetering blade-to-hub attachments, improved ailerons, tip brakes, increased tower heights, aerodynamic tower shapes, variable-speed rotors, and direct-drive transmissions. The new variable-speed wind turbine design allows turbines to more efficiently produce electricity at lower and fast-changing wind speeds. Virtually all existing turbines produce electricity at constant revolutions per minute (rpm) in order to produce 60 Hz ac power. Constant-speed turbines require heavier designs than variable-speed systems because the increase in torque produced by wind gusts must be absorbed by the drivetrain. Newer variable-speed turbines, in which a variable speed drive is provided to accommodate changes in rotor speed due to changes in wind speed and direction while maintaining a constant voltage, constant frequency (60 Hz) output, provide the need for advanced electronic controls with adaptive control capability. Current designs include the use of power electronics (solid-state adjustable speed drive or converter) which allows the speed of the rotor to vary with wind speed while maintaining constant frequency power output. The use of converter technology has improved the total energy capture of the variable speed wind turbine system.
However, such converter systems, while improving the performance and efficiency of wind turbines, do not necessarily operate the turbine for maximum power output. To generate the maximum power output at each varying wind speed is difficult to accomplish. One reason that conventional controllers associated with the variable-speed converter systems do not necessarily optimize power output considerations is the control complexity of meeting all of the potential contradictory demands placed upon, and the large number of variables occurring within, such a system, including, for example, variations in the wind speed and acceleration, wind gusts and wind turbulence. Conventional variable-speed control systems can generate electrical power at a constant frequency in the face of such a plurality of variables, but not in the most efficient manner to produce maximum power output.
Moreover, it is desirable to be able to readily modify or retrofit existing variable speed wind turbine systems with an advanced electronic control system to provide, or improve, power output efficiency.
The present invention utilizes fuzzy logic set theory in an integrated intelligent controller to improve the output power and performance of variable speed wind turbine systems. The aerodynamic efficiency of turbines for varying wind velocity can be maximized by setting the optimum tip speed ratio (ratio of the turbine speed at the blade tip to the free stream wind speed). A typical family of turbine torque-speed curves for varying wind speeds reveals, that for each particular wind speed, there exists a turbine speed to deliver maximum power output. To generate this maximum power at each wind speed, the generator load torque must be matched with the turbine torque. This requires a controller that can vary the turbine speed to get the maximum power output at the given wind conditions. The turbine generator torque is proportional to the square of the rotor speed, while the power output is proportional to the cube of the rotor speed, such that the generator output power is proportional to product of torque and speed. Where constant power hyperbolic curves intersect (at a constant wind speed) the turbine torque/speed curves yields the maximum producible power output by the turbine. When this is plotted, it is to be noted that this value does not occur at the peak of the turbine torque/speed profile.
Because of variable wind speed conditions, including gusts, the turbine generator will be rarely operated at full (rated) load conditions. This means that at any reduced speed, light load steady state condition, the generator efficiency can be further improved by reduction of the generator airgap magnetic flux. At light load conditions, rated flux operation yields excessive core loss and, consequently, low efficiency of operation. The concurrent reduction of flux and increase of active current, so that the developed torque matches the load torque, can provide improved turbine generator efficiency. Thus, once the turbine speed and the developed torque are both set to optimum values, the generator flux component of current can be decreased to reduce the airgap flux until the generator produces the maximum output power.
Fuzzy logic control has been shown to be a promising a technique for extracting maximum performance from modern AC induction motors when the motors are operated at less than rated speeds and loads as set forth in U.S. Pat. No. 5,272,428 of Ronald J. Spiegel and P. Jeffery Chappel, assigned to the same assignee as the present invention and hereby incorporated by reference. Fuzzy logic control has the ability to represent complex systems, such as the wind turbine.
There is, therefore, a need to improve the efficiency of wind turbine systems to increase energy production and hence lower electricity production costs. It is important that the control system to accomplish these objects be relatively inexpensive and capable of addition to existing wind turbine systems through retrofitting, as well as incorporation into the design of new turbine systems.